OK I give up and call upon you auto electrical wizards for your advice, please!

My SE6a has a habit over overheating its coil to the point that it stops working. Changing to a spare coil sorts the problem immediately and the original one can be reused when the next change is needed so no permanent damage.
I have relocated one of the two coils fitted to a cooler area so the heat is not caused by its underbonnet location.

The car is fitted with Lumenition electronic ignition and, on the advice of a friend who had experienced a similar problem on a different car, I changed the Lumenition unit - it made no difference.

I even bought the coil and ballast resistor recommended by Lumenition for their system but then the ballast resistor overheated and fitting the original coil enabled the car to be restarted!

As far as I can see, no ballast resistor is fitted to my car so the coils in regular use are 12v.

The problem is not evident at Motorway cruising speeds but can occur pottering about the countryside or in slow traffic/queues.

Any ideas? Any recommendations of someone who can sort the problem for me?
When the coil is behaving the car is lovely but I could really do with its being reliable!
Thanks
Hugh

Hi Hugh, not often we hear from you so you must be desperate
First of all I must say I have had Lumenition ignition on two Essex engines with no problems.
On my GTC I grilled the coil completely with a similar problem (and Ford Duraspark ignition).
Of course the 6b has the notorious eureka wire instead of a ballast resistor and mine had melted itself into the other wires causing a short circuit.
Are you sure you have no ballast resistor? because I can't see how a 12 volt system could overheat a healthy coil.
Unless ... my memory is hazy but I believe you would have a condenser in the original circuit